Dopamine transporter blockade increases LTP in the CA1 region of the rat hippocampus via activation of the D3 dopamine receptor

Abstract

Dopamine has been demonstrated to be involved in the modulation of long-term potentiation (LTP) in the CA1 region of the hippocampus. As monoamine transporter blockade will increase the actions of endogenous monoamine neurotransmitters, the effect of a dopamine transporter (DAT) antagonist on LTP was assessed using field excitatory postsynaptic potentials recorded in the CA1 region of the rat hippocampal slice preparation. Application of the DAT-specific blocker GBR 12,935 produced a significant enhancement in LTP of Schaffer collateral synapses in the CA1 at concentrations as low as 100 nM. A selective D1/D5 dopamine receptor antagonist (SCH 23,390, 1 microM) did not affect the ability of GBR 12,935 to enhance LTP, whereas application of the D3 dopamine receptor antagonist U 99,194 (1 microM) blocked the GBR 12,935-induced enhancement in LTP. In addition, a D3 dopamine receptor agonist (7-OH-DPAT, 1 microM) caused a significant increase in LTP, an effect that was also blocked by U 99,194 (3 microM). These results suggest that either endogenously released dopamine (facilitated by DAT blockade) or exogenously applied dopamine agonist can act to increase LTP in the CA1 of the hippocampus via activation of the D3 subtype of dopamine receptor.

Figure 1.

GBR 12,935 enhances LTP in a dose-dependent manner. (A) Summary plot of normalized fEPSP slope measurements recorded in the CA1 region of the hippocampus. The closed circles are from GBR 12,935 (1 μM) treated slices; the open circles show results from control slices. Error bars are ±SEM. Three (100 Hz)/(1 sec) stimulus trains separated by 20 sec were used to tetanize the slices at t = 45 min. Insets are 50-msec sweeps taken from representative experiments illustrating the average fEPSP 0–5 min prior to and 25–30 min post-tetanus (the vertical scale bar is 3.5 mV). The left pair of sweeps is from a GBR 12,935-treated slice; the right pair is from a non-drug, control slice. (B) Summary quantification of the dose response for the effect of GBR 12,935 on LTP against control; *P < 0.05; **P < 0.01; ***P < 0.001. Bars are the mean ± SEM.

Figure 2.

GBR 12,935 enhances nmdaLTP. Summary plot of normalized fEPSP slope measurements recorded in the presence of the VDCC antagonist nifedipine (25 μM). The closed circles are results obtained from nifedipine + GBR 12,935-treated slices; the open circles show results from nifedipine-alone treated slices. Error bars are ±SEM. Three (100 Hz)/(1 sec) stimulus trains separated by 20 sec were used to tetanize the slices at 45 min. Insets are 50-msec sweeps taken from representative experiments illustrating the average fEPSP 0–5 min prior to and 25–30 min post-tetanus (the vertical scale bar is 3.5 mV). The left pair of sweeps is from a nifedipine + GBR 12,935-treated slice; the right pair is from a Nifedipine-alone treated slice.

Figure 3.

GBR 12,935 does not affect vdccLTP. Summary plot of normalized fEPSP slope measurements recorded in the presence of the NMDAR antagonist D-APV (50 μM). The closed circles are results obtained from D-APV + GBR 12,935-treated slices; the open circles show results from D-APV-alone treated slices. Error bars are ±SEM. Four 200-Hz, 0.5 sec duration trains at double stimulus intensity separated by 5 sec were used to tetanize the slices at 45 min. Insets are 50-msec sweeps taken from representative experiments illustrating the average fEPSP 0–5 min prior to and 25–30 min post-tetanus (the vertical scale bar is 2.5 mV). The left pair of sweeps is from a D-APV + GBR 12,935-treated slice; the right pair is from a D-APV-alone treated slice.

Figure 4.

GBR 12,935 does not affect LTP of the NMDAR response. Summary plot of normalized fEPSP peak amplitude measurements recorded in the presence of the AMPAR antagonist DNQX (10 μM), low Mg²⁺ (0.1 mM), and the VDCC antagonist nifedipine (25 μM). The closed circles are the results from GBR 12,935-treated slices; the open circles show results from control slices. Error bars are ±SEM. Three 100-Hz trains separated by 20 sec were used to tetanize the slices at 45 min. Insets are 50-msec sweeps taken from representative experiments illustrating the average fEPSP 0–5 min prior to, 25–30 min post-HFS, and 40–45 min post-HFS (50 μM D-APV added at 30 min post-HFS). The vertical scale bar is 1.5 mV. The left set of sweeps is from a DNQX/nifedipine + GBR 12,935-treated slice; the right set is from a DNQX/nifedipine-alone treated slice.

Figure 5.

The enhancement of LTP by GBR 12,935 is prevented by a D3 antagonist. (A) Summary plot of normalized fEPSP slope measurements recorded in the CA1 region of the hippocampus. The closed circles are from SCH 23,390 + GBR 12,935 (1 μM + 1 μM) treated slices; the open circles show results from U 99,194 + GBR 12,935 (1 μM + 1 μM) treated slices. Error bars are ±SEM. Three (100 Hz)/(1 sec) stimulus trains separated by 20 sec were used to tetanize the slices at t = 75 min. Insets are 50-msec sweeps taken from representative experiments illustrating the average fEPSP 0–5 min prior to and 25–30 min post-HFS (the vertical scale bar is 3.5 mV). The left pair of sweeps is from a SCH 23,390 + GBR 12,935-treated slice; the right pair is from a U 99,194 + GBR 12,935-treated slice. (B) Summary quantification of drug effects on LTP. LTP at 30 min post-tetanus is significantly enhanced in slices treated with SCH 23,390 + GBR 12,935 as compared to the U 99,194 + GBR 12,935-treated slices; *P < 0.05 (control data from Fig. 1B are illustrated for comparison).

Figure 6.

7-OH-DPAT enhances LTP, an effect that is prevented by a D3 antagonist. (A) Summary plot of normalized fEPSP slope measurements recorded in the CA1 region of the hippocampus. The closed circles are from 7-OH-DPAT (1 μM) treated slices; the open circles show results from U 99,194 + 7-OH-DPAT (3 μM + 1 μM) treated slices. Error bars are ±SEM. Three (100 Hz)/(1 sec) stimulus trains separated by 20 sec were used to tetanize the slices at t = 75 min. Insets are 50-msec sweeps taken from representative experiments illustrating the average fEPSP 0–5 min prior to and 25–30 min post-HFS (the vertical scale bar is 3.5 mV). The left pair of sweeps is from a 7-OH-DPAT-treated slice; the right pair is from a U 99,194 + 7-OH-DPAT-treated slice. (B) Summary quantification of drug effects on LTP. LTP at 30 min post-tetanus is significantly enhanced in slices treated with 7-OH-DPAT as compared to the U 99,194 + 7-OH-DPAT-treated slices; *P < 0.05 (control data from Fig. 1B are illustrated for comparison).